While devices in accordance with the invention are well adapted for wide use in a wide range of systems, their novel features and advantages are best illustrated by thier use in such exhaust systems as those of steam-operated power plants and the like where the cross sectional dimensions of the exhaust ducts are large and the exhaust gasses must be processed as by precipitators before they can be released through a stack.
Precipitators must be serviced from time-to-time to maintain their efficiency. Exhaust ducts typically are divided to provide parallel branches with a precipitator in each of them. As each such branch duct must be opened by workmen in order that its precipitator may be serviced by workmen, means, commonly called dampers, are employed to enable the precipitators in each duct to be isolated in order to prevent the exposure of the workmen to exhaust gasses as the other branch duct or ducts must be used while a damper isolated precipitator is being serviced as otherwise the associated furnace or furnaces would have to be shut down. Many such ducts are of such dimensions that they may be entered by the workmen.
Such dampers are, except for their size, similar, in general, to gate valves in that a blade within a bonnet is advanced transversely of a duct to block flow therethrough with means provided to seal the blade. In exhaust systems of the type referred-to, the gasses are decidedly noxious and the dampers, accordingly, must insure zero percent leakage. In turn, this required that the sealing means and the blade be free of encrustment or dust and that such debris as well as fluids be prevented from entering the bonnet either to clog the mechanism or to be the source of corrosion or escaping through the bonnet unless it is sealed.
Seals for flue duct dampers are expansible and of a thin metal construction capable of withstanding heat and corrosion. As far as I am aware, the sealing seats of dampers, prior to the present invention were not themselves sealed against becoming clogged when the damper blades were in thier bonnets nor so withdrawn from the path of the blades as to ensure against injury by the blades as they were moved between their two positions.